This invention relates to melt processes for the production of metal ingots, powders, and articles made therefrom, and, more particularly, to the design of a water-cooled refining hearth that aids in the removal of solid foreign matter from the metal.
An increasingly important method for the fabrication of metallic articles for critical applications is powder processing. In one approach, fine powder particles of the metallic alloy of interest are first formed. The proper quantity of the particles is placed into a mold or container and compacted by hot or cold isostatic pressing, extrusion, or other means. In another approach, the final product can be produced to virtually its final shape, so that little or no final machining is required, by depositing a fine spray of the metallic alloy onto a substrate to gradually build up an ingot or article. These powder metallurgical approaches have the important advantage that the microstructure of the product is typically finer and more uniform than that produced by conventional techniques.
The prerequisite to the use of powder fabrication technology is the ability to produce a "clean" powder of the required alloy composition and quality on a commercial scale. (The term "clean" refers to a low level of particles of foreign matter in the metal.) Numerous techniques have been devised for clean powder production. In the melt atomization process, a melt of the alloy of interest is formed, and a continuous stream of the alloy is produced from the melt. The stream is atomized into a fine spray by a gas jet or a spinning disk. Where metal powders are to be produced, the resulting droplets solidify during flight into particles that are collected and graded for size. Particles that meet the size specifications are retained, and those that do not are recycled through the system for remelting and reprocessing. Where an ingot is to be gradually built up, the droplets are deposited upon a substrate to form the ingot which is then further processed.
For some applications the amount of solid foreign matter in the melt must be minimized. The use of water-cooled ("cold") metallic hearths to melt the metallic alloy avoids the introduction of foreign matter such as ceramic particles into the melt. However, when the metallic alloy is melted in a water-cooled hearth, there is typically some amount of oxide or other solid foreign matter present in and on the surface of the melt. This solid matter may be oxide formed by the reaction of the metal with oxygen in the atmosphere, ceramic entrained in the melt in prior processing operations, compounds, or other matter. Some or all of the solid foreign matter may be swept along with the melt into the atomization apparatus, resulting in the inclusion of the foreign matter in the metal spray produced by atomization. The foreign matter is processed into the final articles along with the metal, and incorporated into the articles.
The presence of the foreign matter is usually deleterious to the properties of the final articles produced by this technique. The foreign matter can either initiate cracks or assist their propagation, leading to premature failure of the article. Since the foreign matter cannot be readily removed from the powder mix or the articles, it is important to prevent the foreign matter from being incorporated into the final articles.
There are two possible approaches to preventing foreign matter from entering the final articles. One is to prevent the introduction of the foreign matter into the metal entirely, and the other is to remove the foreign matter after it is present.
Conventional practice achieves a good degree of cleanliness, but it is unrealistic to expect that all foreign matter can be excluded from the metallic alloy at early stages of its production. Commercial melters follow the best cleanliness practices available, but inevitably oxides form on the surface of the melt and other foreign matter finds its way into the melt. The present invention therefore focuses on removing the solid foreign matter from the melt in a refining system prior to introduction of the melt into the atomization process.
Various techniques such as filtering the melt have been utilized in the past to remove solid foreign matter from the melt, but they have not been entirely successful. There is therefore a need for improvements to the techniques for removing solid foreign matter from melts prior to the atomization process. The present invention fulfills this need, and further provides related advantages.